Description of the Prior Art
The invention relates to an intrinsically safe telephone network.
An intrinsically safe telephone network comprises two parts, one of which, which comprises mainly the telephone set and the line connected thereto, can be placed in a dangerous atmosphere without any specific care, that is the currents, voltages and impedances of the line and the telephone set are such that they are not able in any case to cause a spark capable of triggering an explosion in an explosive gas. Such intrinsically safe telephone networks apply more particularly to utilization in mines such as charcoal mines wherein fire-damp explosions are to be avoided.
One of the main difficulties of concern with the intrinsically safe circuits is the fact that all the electrical signals circulating in the line should be at a relatively low level. As a consequence, the attenuation on the line is critical where, for example in case of charcoal mines, the telephone lines in the galleries may have lengths in the range of some tens of kilometers.
French Pat. No. 1,406,586 aims to solve this problem of the line attenuation by arranging at each of the telephone set a local battery feeding amplifier and oscillator circuits. Accordingly, relatively complex circuits were present in the telephone set.